This invention relates to an Fexe2x80x94Crxe2x80x94Ni alloy which is required to be nonmagnetic and is used in electron gun electrodes, and specifically relates to an Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes and Fexe2x80x94Crxe2x80x94Ni alloy sheet for electron gun electrodes made therefrom, with improved press forming properties for drawing.
In general, electron gun electrodes used in color cathode ray tubes and the like are produced by drawing a nonmagnetic Fexe2x80x94Crxe2x80x94Ni stainless steel material with a thickness of 0.1 to 0.7 mm into a predetermined shape using press forming. In order to improve the drawing properties, in particular, to facilitate burring (working in which a circular hole is formed and the circumference thereof is cylindrically projected), improvement in degree of rolling reduction and annealing conditions has been proposed in Japanese Patent Application, First Publication, No. 257253/94. Japanese Patent Application, First Publication, No. 205453/96 proposes a method in which press forming properties are improved by limiting center-line mean roughness and the maximum height of surface roughness in press forming using a low viscosity lubricating oil, which is easy to be removed by degreasing and has been used to increase production efficiency. Japanese Patent Application No. 283039/97 demonstrates that burrs remaining in press punching a through hole relates to cracks in burring, and proposes a method in which burring properties are improved by suitable amounts of S being contained to improve punching properties and in which minute amounts of the elements are controlled to improve the drawing properties.
According to the rapid advances for finer and brighter cathode ray tubes for computers in recent years, requirements on focusing characteristics of the electron guns has become more severe. Therefore, the requirements on materials requires not only high precision formability for the large diameter lens electrodes but also good formability for high speed press forming. However, the prior art alloys have not been adequate since cracks occur at drawing surfaces.
The present invention has been made to respond to the above situation. An object of the invention is to provide an Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes, having superior drawing properties, which have been more severe in recent years, in particular, which can inhibit the occurrence of cracks in drawing.
The inventors have intensively researched the surface conditions of materials to solve the problems. As a result, the inventors have found that the drawing properties are influenced by the size and the number of groups of inclusions existing in a surface layer of a material. In particular, they have found that in groups of inclusions (including single inclusion) existing in a surface layer, ones with certain size or more influence the occurrence of cracks in drawing, and they have been able to inhibit the occurrence of cracks by reducing these inclusions. FIG. 1 is a diagram showing the relationship between the number of groups of inclusions existing in a surface layer of an Fexe2x80x94Crxe2x80x94Ni alloy with a thickness of 0.6 mm and the incidence of cracks. It should be noted that the incidence of cracks was obtained by sampling 200 pieces at random from 2000 pieces of punched samples for inspection.
That is, the groups of inclusions were classified by the width and the length of the groups of widths of 5 xcexcm or more and less than 10 xcexcm and with lengths of 20 xcexcm or more, with widths of 10 xcexcm or more and less than 20 xcexcm and with lengths of 20 xcexcm or more, and with widths of 20 xcexcm or more and with lengths of 20 xcexcm or more, and the number of the groups of inclusions and the incidence of cracks with respect to each classification were plotted in FIG. 1. It is shown in FIG. 1 that the groups of inclusions, with widths of 5 xcexcm or more and less than 20 xcexcm and with lengths of 20 xcexcm or more, do not relatively influence the occurrence of cracks in drawing even if the number thereof per unit area increases.
In contrast, in the case of the groups of inclusions, with widths of 10 xcexcm or more and less than 20 xcexcm and with lengths of 20 xcexcm or more, the incidence of cracks exceeds 1% when the number of the groups nearly exceeds 20/mm2, and the incidence of cracks rapidly increases as the number of groups increase further. In the case of groups of inclusions, with widths of 20 xcexcm or more and with lengths of 20 xcexcm or more, the incidence of cracks exceeds 1% when the number of the groups nearly exceeds 5/mm2, and the incidence of cracks rapidly increases as the number of groups further increase. This shows that the occurrence of cracks in drawing can be inhibited by restricting the groups of inclusions such that the groups of lining inclusions, with widths of 10 xcexcm or more and less than 20 xcexcm and with lengths of 20 xcexcm or more, is 20/mm2 or less, the groups of lining inclusions, with widths of 20 xcexcm or more and with lengths of 20 xcexcm or more, is 5/mm2 or less.
Furthermore, according to the research by the inventors, it has been demonstrated that the incidence of cracks may exceed 1% when the inclusions are Al2O3 or composite inclusions of MnO and SiO2 even if the number and the size of the groups of inclusions are restricted as above, and that the probability of cracks in drawing changes according to the chemical composition of the inclusions.
The number and the size of groups of inclusions in a surface layer of a material can be measured as follows. First, a surface of a material is specularly polished and then electropolished in phosphoric acid so as to facilitate distinction of inclusions. Then, the optical microscopic image of the surface is scanned by an image analyzer, and the images of inclusions are specified using the difference in the color tone between the inclusions and the matrix of the Fexe2x80x94Crxe2x80x94Ni alloy. Then, each image of the inclusions is enlarged 5 xcexcm in the rolling direction and enlarged 5 xcexcm in the transverse direction to the rolling direction, and the image is then reduced 5 xcexcm in the respective directions. By these operations, the inclusions in the image, which exist over short distances, combined with each other into a group. Finally, the width and the length of each group of the inclusions (including single inclusions) are measure by the image analyzer.
The chemical composition of the group of inclusions is obtained by quantitative analysis with an electron beam microanalizer of ten inclusions chosen randomly.
The Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes of the invention has been made based on the above knowledge, and is characterized in comprising: 15 to 20% Cr; 9 to 15% Ni; 0.12% or less C; 0.005 to 1.0% Si; 0.005 to 2.5% Mn; 0.03% or less P; 0.0003 to 0.0100% S; 2.0% or less Mo; 0.001 to 0.2% Al; 0.003% or less O; 0.1% or less N; 0.1% or less Ti; 0.1% or less Nb; 0.1% or less V; 0.1% or less Zr; 0.05% or less Ca; 0.02% or less Mg by weight; balance Fe; and inevitable impurities; wherein when the alloy is rolled into a sheet with a thickness in the range of 0.1 to 0.7 mm, the surface portion of the sheet includes groups of lining inclusions, the number of groups with widths of 10 xcexcm or more and less than 20 xcexcm and with lengths of 20 xcexcm or more is 20/mm2 or less, and the number of groups with widths of 20 xcexcm or more and with lengths of 20 xcexcm or more is 5/mm2 or less.
According to the preferred embodiment of the invention, the above Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes may be specified by the chemical composition of inclusions in 40xe2x89xa7SiO2xe2x89xa7100, 0xe2x89xa7Al2O3xe2x89xa740, and 0xe2x89xa7MnOxe2x89xa730 by atomic %.
Furthermore, the invention provides an Fexe2x80x94Crxe2x80x94Ni alloy sheet for electron gun electrodes obtained by rolling the above Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes to a thickness in the range of 0.1 to 0.7 mm.
In the following, the reasons for the above numerical limitations will be explained. In the following explanation, xe2x80x9c%xe2x80x9d means xe2x80x9cweight %xe2x80x9d.
(Cr): Electron gun electrodes are required to be nonmagnetic, and the magnetic permeability thereof is required to be 1.005 or less to be nonmagnetic. In order to meet this requirement, the content of Cr is restricted to within the range of 15 to 20%. The Cr content is preferably in the range of 15 to 17%.
(Ni): If the content of Ni is less than 9%, magnetism is excessively imparted. If the Ni content is more than 15%, the material cost is relatively high. Hence, the Ni content is restricted to within the range of 9 to 15%.
(C): If the content of C is more than 0.12%, carbides excessively precipitate and drawing properties are inferior. Hence, the C content is restricted to 0.12% or less.
(Si): Si is added for deoxidation. If the Si content is less than 0.005%, the effect as a deoxidizer cannot be obtained. On the other hand, if the Si content is more than 1.0%, the formability is inferior. Hence, the Si content is restricted to within the range of 0.005 to 1.0%.
(Mn): Mn is added for deoxidation and formation of MnS. If the Mn content is less than 0.005%, these effects are not expected. If the Mn content is more than 2.5%, the hardness of the alloy markedly increases, whereby the drawing properties are inferior. Hence, the Mn content is restricted to within the range of 0.005 to 2.5%.
(P): If the P content is more than 0.03%, the drawing properties are very inferior. Hence, the P content is restricted to 0.03% or less.
(S): When contained in an appropriate amount, S forms MnS together with Mn, thereby inhibiting formation of burrs in press punching holes and generation of burring cracks in burring. If the S content is less than 0.0003%, such effects are not expected. If the S content is more than 0.0100%, coarse MnS is formed, whereby the drawing properties are inferior. Hence, the S content is restricted to within the range of 0.0003 to 0.0100%.
(Mo): Since Mo improves corrosion resistance, Mo may be advantageously added when special corrosion resistance is required. However, if the Mo content is more than 2.0%, the formability is inferior. Hence, the Mo content is restricted to 2.0% or less.
(Al): Al is added for deoxidation. If the Al content is less than 0.001%, the effect as a deoxidizer cannot be obtained. On the other hand, if the Al content is more than 0.2%, the formability is inferior. Hence, the Al content is restricted to within the range of 0.001 to 0.02%.
(O): When a large amount of O is contained, the amount of oxide-type inclusions increase, whereby drawing properties are inferior. Hence, the O content is restricted to 0.005% or less.
(N): If the N content is more than 0.1%, the formability is inferior. Hence, the N content is restricted to 0.1% or less.
(Ti): Ti forms carbides, sulfides, oxides and nitrides, whereby the drawing properties are inferior. Hence, the Ti content is restricted to 0.1% or less. A more preferable range for the Ti content is 0.02% or less.
(Nb): Nb forms carbides, sulfides, oxides, and nitrides, whereby the drawing properties are inferior. Hence, the Nb content is restricted to 0.1% or less. A more preferable range of the Nb content is 0.02% or less.
(V): V forms carbides and nitrides, whereby the drawing properties are inferior. Hence, the V content is restricted to 0.1% or less. A more preferable range for the V content is 0.02% or less.
(Zr): Zr forms sulfides and oxides, whereby the drawing properties are inferior. Hence, the Zr content is restricted to 0.1% or less. A more preferable range for the Zr content is 0.02% or less.
(Ca): Ca forms sulfides and oxides, whereby the drawing properties are inferior. Hence, the Ca content is restricted to 0.05% or less. A more preferable range of the Ca content is 0.01% or less.
(Mg): Mg forms oxides, whereby the drawing properties are inferior. Hence, the Mg content is restricted to 0.02% or less. A more preferable range of the Mg content is 0.005% or less.
(Number of Groups of Inclusions in Surface Layer)
If groups of lining inclusions with widths of 10 xcexcm or more and less than 20 xcexcm and with lengths of 20 xcexcm or more exist at more than 20/mm2 in a surface layer of a sheet, cracks in drawing readily occur, and the limitations are therefore determined. For the same reasons, the number of groups of inclusions with widths of 20 xcexcm or more and with lengths of 20 xcexcm or more is restricted to 5/mm2.
(Composition of Inclusions)
If the amount of Al2O3 in the chemical composition of inclusions is large, cracks in drawing readily occur. Moreover, if the chemical composition of inclusions is a MnO rich composite inclusions of MnO and SiO2, or alternatively, a composite inclusions of MnO and Al2O3, cracks in drawing readily occur. Therefore, the amounts of MnO and Al2O3 in the chemical composition of inclusions should be restricted. Hence, the inclusions preferably comprise 40xe2x89xa7SiO2xe2x89xa7100, 0xe2x89xa7Al 2O3xe2x89xa740, and 0MnOxe2x89xa730 by atomic %.